This invention pertains generally to nuclear reactor flux monitors, and more particularly to flux deviation monitors employed to alarm radial flux maldistributions within the core of a nuclear reactor.
Recent developments in pressurized water reactors, when considered in combination, may compromise the ease of plant operation or operation at the maximum permitted power level. These developments require decreased margins of the design hot channel factors and have established an awareness that the percent increase in hot channel factors per percent increase in indicated radial tilt can be larger than assumed in the design phase of a plant. The aforegoing, as determined by reactor plant start-up measurements, can require the flux deviation alarm setpoint, which is employed to annunciate radial power maldistributions, to be set at such a low level that spurious alarms can be tripped due to normal instrumentation drift.
The hot channel factors are design parameters of the core which include: the heat flux hot channel factor, F.sub.Q.sup.T, which is defined as the maximum local heat flux on the surface of a fuel rod divided by the average fuel rod heat flux; the nuclear heat flux hot channel factor, F.sub.Q.sup.N, which is defined as the maximum local fuel rod linear power density divided by the average fuel rod linear power density; the engineering heat flux hot channel factor, F.sub.Q.sup.E, which is the allowance on heat flux required for manufacturing tolerances to compensate for local variations in enrichment, pellet density and diameter, surface area of the fuel rod and eccentricity of the gap between the pellet and the cladding; the nuclear enthalpy rise hot channel factor, F.sub..DELTA.H.sup.N, which is defined as the ratio of the integral of linear power along the rod with the highest integrated power to the average rod power; and the engineering nuclear enthalpy rise hot channel factor, F.sub..DELTA.H.sup.E, which is the allowance on enthalpy rise required for the effects of flow conditions and fabrication tolerances on the hot channel enthalpy rise. It can therefore be appreciated that the total heat flux hot channel factor, F.sub.Q.sup.T, is the product of F.sub.Q.sup.N and F.sub.Q.sup.E, and the total enthalpy rise hot channel factor, F.sub..DELTA.H.sup.T, is the product of F.sub..DELTA.H.sup.N and F.sub..DELTA.H.sup.E.
The aforegoing design parameters, which will govern to a degree the limits of reactor plant operation, are initially calculated using analytical models and confirmed or modified in accordance with the results obtained from in-core flux monitoring measurements performed during reactor testing at start-up using movable in-core flux monitoring systems such as the system described in Application Ser. No. 379,159, entitled "A Method of Automatically Monitoring The Power Distribution Of A Nuclear Reactor Employing Movable In-Core Detectors", by James J. Loving, Jr., filed July 13, 1973. During reactor plant operation, operation within design limits is assured by constant surveillance of the in-core power distribution using the excore power range detector system. This system provides information on the axial and radial core power distribution and alarm maldistributions which are commonly referred to as tilts.
One type of flux deviation alarm system presently in use employs four excore detectors which are equidistantly positioned around the periphery of the core, external of the reactor pressure vessel. Thus, each detector provides flux information on a corresponding quadrant of the core. In addition, each detector is divided into an upper and lower section which respectively provide corresponding flux data for the upper and lower sections of the corresponding monitored quadrant. The detector outputs are communicated to a flux deviation alarm circuit which annunciates an alarm when a given quadrant section of the core exhibits a positive flux deviation from the average flux level for the corresponding half of the core. That is, an alarm is annunciated when any upper detector channel experiences a high positive flux deviation from the average of the upper detector channels, or when any lower detector channel experiences a high positive flux deviation from the average of the lower channels. The alarms are arranged to trip over the range of 50-120% of rated power and are cut off below 50%. The alarm can be set to trip on a deviation of from 0 to 20%.
The flux deviation alarm setpoints are calculated from the design and start-up information and set at a low level to alert the reactor plant operator to an abnormal quadrant power tilt in time to take corrective action. The object is to insure that the steady state hot channel factors are within the design and safety limits. However, as previously stated, recent developments in pressurized water reactors have required the alarm setpoint to be set at such a low level that spurious alarms are likely due to normal instrumentation drift.
In addition to the susceptibility to spurious alarms, the present system is relatively insensitive to core radial maldistributions within proximity of the minor axis of the core which divides the core quadrants. This means that while the measured tilt may be within design limits, maldistributions within the area of the minor axis can exceed design limits.
Accordingly, an improved method and apparatus is desired that can more sensitively monitor axial flux deviations within the core. In addition, the method and apparatus desired should have a low susceptibility to spurious alarms.